Charging is a suitable means for increasing the power of an internal combustion engine with unchanged engine displacement or for reducing the engine displacement with constant power. Charging leads to an increase of the power/volume ratio and to a more favorable performance measure. The air for the combustion process is compressed, whereby a greater air mass may be fed to each cylinder per working cycle. The fuel mass and therefore the mean pressure may thus be increased.
If the engine displacement is reduced, the load configuration may shift toward higher loads, at which the specific fuel consumption is lower. By charging in combination with a suitable transmission, what is known as a downspeeding may also be provided, with which lower specific fuel consumption likewise may be obtained.
The charging consequently assists the ongoing effort in the development of internal combustion engines to minimize the fuel consumption, so as to enhance the efficacy of the internal combustion engine.
In one example, at least one exhaust gas turbocharger may be used for the charging, in which a compressor and a turbine are arranged on the same shaft. The hot exhaust gas flow is fed to the turbine and expands with energy release in the turbine, whereby the shaft is set in rotation. The energy delivered from the exhaust gas flow to the turbine and ultimately to the shaft is used for the drive of the compressor, which likewise is arranged on the shaft. The compressor conveys and compresses the charge air fed thereto, whereby a charging of the cylinders is achieved. A charge air cooler is advantageously provided downstream of the compressor in the intake system and is used to cool the compressed charge air prior to entry into the at least one cylinder. The cooler lowers the temperature and thus increases the density of the charge air, such that the cooler also contributes to an enhanced filling of the cylinders with a greater air mass. Compression is achieved by cooling.
In the exhaust gas turbocharger, unlike a mechanical supercharger, no mechanical connection for the power transfer between supercharger and internal combustion engine is needed. The mechanical supercharger draws the energy for drive thereof from the internal combustion engine and thus reduces the provided power and in this way diminishes the efficacy, while the exhaust gas turbocharger utilizes the exhaust gas energy of the hot exhaust gases.
The mechanical supercharger, unlike the exhaust gas turbocharger, generally may generate and provide the charge air pressure independently of the momentary operating state of the internal combustion engine, in particular even at low speeds of rotation of the crankshaft. This is true in particular for a mechanical supercharger which may be driven by means of electric machine.
In the case of exhaust gas turbocharging, difficulties are often encountered, specifically in providing a sufficiently high charge air pressure at low rotational speeds as well. When a certain rotational speed is undershot, a torque reduction is observed. This torque reduction is understandable as the charge air pressure ratio is dependent on the turbine pressure ratio. In one example, reduction of the rotational speed leads to a smaller exhaust gas mass flow and therefore to a smaller turbine pressure ratio. As a result, the charge air pressure ratio likewise decreases toward excessively low rotational speeds, which is equivalent to a torque reduction.
Previous attempts to enhance the torque characteristic of a charged internal combustion engine include a small turbine cross section with exhaust gas blow off. A turbine of this type is also referred to as a wastegate turbine. If the exhaust gas volume exceeds a threshold value, some of the exhaust gas flow is guided past the turbine via a bypass line within the scope of what is known as exhaust gas blow off. However, the charging described above may be insufficient at higher rotational speeds or with greater exhaust gas volumes. In addition, the blown-off exhaust gas may be guided past the turbine without further use, without utilization of the energy available in the hot exhaust gas.
The torque characteristic of a charged internal combustion engine may also be enhanced by means of a plurality of turbochargers arranged in parallel, for example, by means of a plurality of turbines of smaller turbine cross section arranged in parallel, the turbines being connected successively with increasing exhaust gas volume.
The torque characteristic may also be influenced by means of a plurality of exhaust gas turbochargers connected in series. As a result of the connection series of two exhaust gas turbochargers, of which one exhaust gas turbocharger serves as high-pressure stage and one exhaust gas turbocharger serves as a low-pressure stage, the compressor map may be extended, both to smaller compressor flows and to larger compressor flows.
In the case of the exhaust gas turbocharger serving as high-pressure stage, a shift of the pump capacity to smaller compressor flows is possible in particular, whereby high charge air pressure conditions may be obtained even with small compressor flows, which significantly enhances the torque characteristic in the lower rotational speed range. This may be achieved by use of the high-pressure turbine for small exhaust gas mass flows and provision of a bypass line, with which exhaust gas may be increasingly guided past the high-pressure turbine with increasing exhaust gas mass flow. The bypass line for this purpose branches off upstream of the high-pressure turbine from the exhaust gas discharge system and leads back into the exhaust gas discharge system upstream of the low-pressure turbine. A shutoff element may be positioned in the bypass line in order to control the exhaust gas flow guided past the high-pressure turbine.
The patent application DE 10050161 A1 describes an internal combustion engine in which the exhaust gas turbocharging is provided with an electric auxiliary drive including a stator and a rotor. The auxiliary drive may be activated by shifting of the rotor, where the rotor is coupled to the compressor impeller for conjoint rotation therewith as a result of the shifting. Alternatively, the rotor may be independent of the actual compressor impeller and may be connected to an upstream precursor wheel for conjoint rotation therewith. The rotor does not drive the exhaust gas turbocharger as such, but rather the precursor wheel, which contributes to the conveyance of charge air at low rotational speeds. In the first case a complex shift mechanism and a clutch or coupling may be provided.
The inventors herein provide a spark-ignited charged internal combustion engine, which is simplified and therefore enhanced in respect of the activation of the electric auxiliary drive of the exhaust gas turbocharging. The inventors also provide a method for operating the spark-ignited internal combustion engine.
In one example, a spark-ignited charged internal combustion engine may include at least one cylinder, an intake system for feeding charge air to the at least one cylinder, an exhaust gas discharge system for discharging exhaust gas from the at least one cylinder, and at least one exhaust gas turbocharger, which comprises a housing, a turbine which is arranged in the exhaust gas discharge system and which has at least one turbine impeller mounted on a rotatable shaft, and a compressor which is arranged in the intake system and which has at least one compressor impeller mounted on the rotatable shaft. The turbocharger may further include an electric auxiliary drive, which comprises a stator and a rotor, the rotor of the electric auxiliary drive comprising a wheel arranged and mounted on the shaft of the exhaust gas turbocharger, said wheel being a wheel running freely in one direction of rotation, which runs freely when the rotational speed nshaft of the shaft of the exhaust gas turbocharger is greater than the rotational speed nwheel of the wheel.
The rotor of the electric auxiliary drive may be mounted on the shaft of the exhaust gas turbocharger via an overrunning clutch, such that the rotor runs freely in one direction of rotation and in the other direction of rotation is connected to the shaft of the exhaust gas turbocharger in a frictionally engaged manner. The rotor may turn freely when the rotational speed nshaft of the shaft of the exhaust gas turbocharger is greater than the rotational speed nwheel of the rotor.
The shaft of the exhaust gas turbocharger may be driven by the turbine of the exhaust gas turbocharger, specifically when enough exhaust gas flows through the turbine and the turbine may perform the compressor work. Then, the rotational speed nshaft of the shaft of the exhaust gas turbocharger may be greater than the rotational speed nwheel of the rotor and the rotor turns freely. The shaft may revolve freely under the rotor. Any braking torques or detent torques of the deactivated auxiliary drive may be harmless and may be without influence. Thus, the exhaust gas turbocharger may be operated with omission of the electric auxiliary drive.
Otherwise, the shaft of the exhaust gas turbocharger is driven by the rotor of the electric auxiliary drive specifically when insufficient exhaust gas flows through the turbine and the turbine may no longer perform the compressor work. This is generally anticipated at low rotational speeds or with small exhaust gas volumes. The rotor is then connected to the shaft of the exhaust gas turbocharger in a frictionally engaged manner, entrains the shaft and allows this to turn at the rotational speed nshaft=nwheel. The electric auxiliary drive in the present case takes over the drive of the exhaust gas turbocharger. This may occur at low rotational speeds or with small exhaust gas volumes.
The above described spark-ignited charged internal combustion engine provides a simplified and efficient system for the activation of the electric auxiliary drive for delivering boost to the engine, especially when exhaust volume (for example, at low engine speeds) is not adequate to provide enough rotational speed through the turbine to the shaft for driving the compressor. The electric auxiliary drive may engage the shaft to increase compression and delivery of more air to the engine through the compressor, thereby providing boost even at low engine speeds, and ensuring a wider operational range of the compressor for delivering adequate charge air to the engine.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.